The present invention generally relates to a printer head which employs a light emitting element array, and more particularly, to a printer head capable of obtaining printer output having high picture quality by diminishing contrast of an image forming light profile.
An image recording device such as a photo printer, an electronic-photo-type copying machine, or the like typically forms an image by repeating processes such as electrification, exposure, development, transfer, cleaning and elimination of electricity with respect to a photosensitive drum. There is now exemplified a printer head which utilizes light emitting diodes (hereinafter referred to as LED) for providing a light signal to control the exposure of the photosensitive drum in accordance with a picture signal.
The LED printer head, as illustrated in FIG. 4(a), includes a plurality of LED elements A which are arranged in an array. Each LED element A has, as illustrated in an enlarged view of FIG. 4(b), a light emitting portion 2 and an electrode 3. The electrode 3 is connected through a bonding pad 3' to one of driver ICs 500 formed on both sides of the LED elements A.
Each driver IC 500 is connected to a control member 600. The driver IC 500 is controlled by the control member 600 according to an image signal inputted to the control member 600 from outside, thereby transmitting a driving signal to the corresponding LED elements. The LED elements thus driven emit light, and the emitted light beams travel through a converging rod lens array (not shown) consisting of SELFOC lenses, or the like disposed above the LED elements and are formed into an exposure image on the photosensitive drum, thus forming a latent image.
There arises, however, such a problem inherent in the printer head using the LED elements that variation in an exposure amount of the photosensitive body is caused by dispersion in LED characteristics or unevenness in an optical system. More specifically, if there is dispersion in the characteristics of LED elements A", as illustrated in FIG. 5, the variation is produced in an image forming light profile E"' which is obtained by converging the emitted light beams through the optical system. As a result, density of a produced picture does not become uniform and also picture quality is deteriorated.
Under such circumstances, there have been made a variety of improvements. For instance, a resistor which can be trimmed is electrically connected to each LED element, and intensities of emitted light beams are controlled by trimming the resistors so as to become uniform (e.g., Japanese Patent Laid-Open Publication No. 112376/1983). Another method is such that the light intensity of a light emitting element or element block is controlled while correcting it with a voltage of a driving signal for the light emitting element, thus stabilizing the light intensity of the light emitting element (e.g., Japanese Patent Laid-Open Publication No. 194566/1984).
Methods of simply increasing the luminance are disclosed in, e.g., Japanese Utility Model Laid-Open No. 159763/1983, Japanese Utility Model Laid-Open No. 138253/1984, Japanese Patent Laid-Open Publication No. 70782/1985, and Japanese Patent Laid-Open Publication No. 61479/1986. There is made, however, no proposal for the purpose of improving the overall light emitting profile when actually incorporating the method into a printer.
The LED elements which constitute the LED printer head are arranged in such a way that, as depicted in FIG. 6(a), the electrode 3 is disposed on the center of the light emitting portion 2, thus forming an LED element A; this LED element A in turn constitutes one dot. In general, 64 dots of the LED elements constitute one chip. The LED chips are dice-bonded at a connecting portion C' so as to form a head having a required length. FIG. 6(b) illustrates the light emitting profile of the LED printer head thus constituted, wherein the light intensity is reduced in the electrode portion 3 and at a boundary B' between the adjacent LED elements, and further at the connecting portion C' between the chips. Especially in the connecting portion C', some deterioration of luminance tends to be caused by cutting damages, resulting in creation of an unfavorable light emitting profile.
An enhancement in light intensity at edge portions of the light emitting portion 2 of the LED element is attributed to the fact that an SiN film employed as a diffusion mask when forming the LED element remains at the edge portions of the light emitting portion 2. Namely, if the light emitting portion 2 of the LED element directly borders on an atmosphere, part of the emitted light is subjected to the total internal reflection at the boundary surface because of the large difference between the refractive indices of the light emitting portion 2 and the atmosphere. Therefore, a transmissivity of the emitted light is decreased. However, by virtue of the presence of the SiN film whose refractive index is approximately in the midst of those of the light emitting portion 2 and the atmosphere, the transmissivity of the edge portion is not decreased so much, thereby resulting in the enhancement in light intensity at that portion.
FIG. 7 shows a profile E' of image forming light which was originally emitted from the LED element array with such a light emitting profile D' as shown in FIG. 6(b) and then transmitted from the converging rod lens array 8 consisting of, e.g., the SELFOC lenses.
In FIG. 7, the reference symbol A' designates the LED element portion; B', the boundary between the adjacent LED elements; and C', the connecting portion between the LED chips. The light intensity is strong at the LED element portion A', whereas it is weak at the boundary B' between the LED elements and at the connecting portion C' between the LED chips, thus forming gorges.
The contrast thereof can be expressed as: ##EQU1## where MAX is the maximum value of the light intensity profile E', and the MIN is the minimum value thereof.
Even if the light intensities of the light emitting portions are made uniform, i.e., if MAXs are made uniform, the contrast of the image forming light profile E' is still large. Therefore, the conventional LED printer head is accompanied with the following defects.
A method of recording the picture image includes a positive developing type background writing method and a negative developing type image writing method. The background writing method is based on the steps of exposing a photosensitive body to the light emitted from the LED elements and eliminating electric charge at the portions excluding a character image to form an electrostatic latent image. The negative developing method is intended to develop the portion written by the light from the LED elements. If the contrast of the image forming light profile is large, there is created such a problem in the former method that black streaks are apt to appear at the portions corresponding to the boundaries (portions B' in FIG. 7) between the LED elements and at the connecting portions (portions C' in FIG. 7) between the LED chips, and the image quality is deteriorated because of bad reproducibility of fine lines.
The reproducibility of fine lines herein implies reproducibility of one-dot lines. In the LED printer based on the background writing method, gorges, viz., portions of MIN are produced in the image forming light profile of the LED printer head at the portions excluding the light emitting portions of the LED elements, especially at the connecting portions between the LED chips. The light of that portion is relatively weak, and hence the electric charge on the photosensitive body can not be fully erased. As a result, a small amount of toner is adhered thereto, and the black streaks are likely to be produced. To eliminate the black streaks, the following method is taken: The light intensity is strengthened by increasing, e.g., the luminance of the LED elements or a lighting duty, thereby erasing the electric charge. As a result, however, one-dot lines tend to become thin, the image reproduction becomes difficult, and the image quality is deteriorated.
This is, as described above, derived from the fact that because of the structural disadvantage of the LED elements it is difficult to form such light having the image forming light profile with the small contrast as is formed in a laser scanning system.
In the image writing method, white streaks are apt to appear on the contrary to the background writing method. To eliminate the white streaks the luminance has to be decreased on the whole. This conduces to a problem in which the one-dot lines become thick and the image quality is degraded.